Study On The Simulation Technique In Self-heating Effect And KNIK Effect Of Thin Film Transistors

Poly-Si TFTs play an important role for LCD fabrications, enhancing the integration of both active matrix and peripheral driving circuitry on the same substrate. However, poly-Si TFTs lead to some poor effects, like self-heating effect and KINK effect which mainly induced by the inherent SOI (silicon on isolator) structure of thin film transistor. For self-heating effect, heat generated in the TFT channel from Joule heating can not be efficiently dissipated to the outside via the glass substrate which have high thermal resistivity. Regarding the KINK effect, there is a floating region in the channel bottom because of the glass substrate. When the device working at a high drain voltage, the impact ionization effect which caused by the strong electric field near the drain electrode occurred. Holes produced by the impact ionization accumulated in the floating area, causes the threshold voltage to drop. On the other hand, because of the increasing channel carriers density induced by impact ionization effect, the channel current continues to increase in the saturation area.The thin film transistor parameters, like the threshold voltage, leakage current, on-state current, the sub-threshold voltage slope, are influenced by Self-heating effect and the KINK effect badly. By using the TCAD tools, we studied the self-heating effect and the KINK effect by establishing the device structure through process simulation software TSUPREM4, calculated its electricity characteristic by device simulation software MEDICI.First, the device with LDD structure was established to study the influence of low drain electric field to these two effects. It is found that the influence of the LDD structure to the temperature effect under unit drain current is not big, but it may reduce the KINK effect, and when the LDD implant parameters (for example implant dose, implant energy, LDD length) are different, the KINK effect correspondingly enhanced or weakened. The appropriate LDD parameters which may depress the KINK effect are provided based on the simulated results of TCAD tools. Then, regarding the self-heating effect, the influence of temperature to the device performance was discussed, as well as the influence of device parameters to the self-heating effect. Several essential factors that influenced the self-heating effect was given. According to the simulation, with the self-heating effect, the threshold voltage was reduced, the leakage current was increased, the sub-threshold voltage slope was added, and the drain current continues to increase in the saturation region. It concluded that it is the channel thickness, the substrate thickness and the substrate material that dominated the self-heating effect of the thin film transistor. To the KINK effect, the influence of the KINK effect to the device performance and the influence of several parameters to the KINK effect were discussed. The simulation indicated that the output characteristic curve was warped in the saturation region and the drain conductance was unstable.